1. Field of the Invention
The present invention relates to an ultrasound transducer element (hereinafter also referred to an “element”) provided with a cell group constituted by a plurality of capacitive ultrasound transducer cells (hereinafter also referred to as “cells”), and an ultrasound endoscope provided with an insertion portion with the ultrasound transducer element arranged at a distal end portion.
2. Description of the Related Art
An ultrasound diagnostic method of radiating an ultrasound to an inside of a body and performing imaging of a state inside the body from an echo signal to make a diagnosis has been spreading. One of medical apparatuses used for the ultrasound diagnostic method is an ultrasound endoscope. In the ultrasound endoscope, an element is arranged at a distal-end rigid portion of an insertion portion to be introduced into an inside of a body. The element has a function of converting an electrical signal to an ultrasound and transmitting the ultrasound into the body, and a function of receiving an ultrasound reflected inside the body and converting the ultrasound to an electrical signal.
For a lot of today's elements, ceramic piezoelectric materials including lead, which is environmentally very hazardous, for example, PZT (lead titanium zirconium oxide), are mainly used. On the other hand, development of an element manufactured with the use of MEMS (micro electro mechanical systems) has been advanced, the element having a plurality cells constituted by capacitive micro-machined ultrasonic transducers (hereinafter referred to as “c-MUTs”) the material of which does not include lead.
For example, an element 120 shown in FIGS. 1 to 3 is disclosed in specification of U.S. Pat. No. 6,854,338. As shown in FIG. 1, which is a top view, the element 120 has cells 110 constituted by twenty-five c-MUTs, which are a base unit of ultrasound transmission/reception.
FIG. 2 is a cross-sectional view of one of the cells 110 of the element 120. FIG. 3 is a partially exploded view of four of the cells 110 of the element 120 shown in FIG. 1, and a regular square indicated by broken lines shows an area exclusive to one cell 110, in other words, a plane view shape of the cell 110. Note that the plane view shape will be hereinafter referred to simply as a shape below. The shape of the cell 110 can be regarded as a regular square.
As shown in FIGS. 2 and 3, the cell 110 has a conductive substrate 111, which is also a lower electrode 112, and an upper electrode 116 arranged facing the substrate 111, with a cavity 114H therebetween. An area of the upper electrode 116 immediately above the cavity 114H constitutes a membrane 118 which ultrasonically vibrates. The cavity 114H is formed by a through hole formed in an insulating layer 114. The cavity 114H is a sealed space isolated from an outside.
When a drive signal is applied between the lower electrodes 112 and the upper electrodes 116, the membranes 118 vibrate, and the cells 110 generate an ultrasound. When an ultrasound comes in from the outside, the ultrasound is converted to an electrical signal by utilizing a fact that the membranes 118 are deformed, and capacitance between the electrodes changes. The larger an aperture ratio indicated by “(an area of the membranes 118)/(an area of the cells 110)”, the higher a transmission/reception sensitivity of the element 120 is.
In the element 120 having the regularly square cells 110, areas other than areas of the membranes 118 immediately above the circular cavities 114H are non-sensitive areas which do not contribute to transmission/reception of an ultrasound. For example, capacitance between the electrodes in the non-sensitive areas is so-called parasitic capacitance which does not change at a time of receiving an ultrasound.